The present invention relates to transmission of reference signals in an Orthogonal Frequency Division Multiplex (OFDM)-based system, and more particularly to transmission of demodulation reference signals in an OFDM-based communication system.
In the Long-Term Evolution (LTE) mobile communication system defined by the 3rd Generation Partnership Project (3GPP), uplink radio transmissions utilize Discrete Fourier Transform (DFT)-spread-OFDM (DFTS-OFDM) techniques. FIG. 1a is a block diagram illustrating how DFTS-OFDM works. Blocks of M modulation symbols 101 are first applied to a size M DFT 103. The output of the DFT 103 is then supplied to a frequency mapper 105, which maps the DFT output to selective consecutive inputs of a size N Inverse DFT 107 that can, for example, be implemented by means of Inverse Fast Fourier Transform (IFFT) processing. By adjusting the block size M, the instantaneous bandwidth of the transmitted signal can be varied. Similarly, by adjusting (e.g., shifting) the set of IFFT inputs to which the DFT output block of size M is mapped, the frequency-domain position of the transmitted signal can be adjusted. DFTS-OFDM can be thought of as an OFDM transmission (an IFFT) preceded by a DFT-based pre-coding. Thus, as with OFDM, the spectrum of a DFTS-OFDM signal can be seen as consisting of a number of subcarriers.
In some other mobile communication standards, pure OFDM is used instead of DFTS-OFDM. FIG. 1b is a block diagram illustrating how pure OFDM works. Blocks of M modulation symbols 111 are applied directly to a frequency mapper 113, which maps the M modulation symbols to selective consecutive inputs of a size N Inverse DFT 115 that can, for example, be implemented by means of Inverse IFFT processing. By adjusting the block size M, the instantaneous bandwidth of the transmitted signal can be varied. Similarly, by adjusting (e.g., shifting) the set of IFFT inputs to which the DFT output block of size M is mapped, the frequency-domain position of the transmitted signal can be adjusted. As mentioned above, the spectrum of an OFDM signal can be seen as consisting of a number of subcarriers.
FIG. 2 is a diagram illustrating a basic subframe structure of an LTE uplink radio interface. It will be appreciated that aspects of the LTE system are presented here to facilitate an understanding of various aspects of the invention. However, characteristics of the LTE system that make it a suitable environment for practicing the invention are also present in other systems (e.g., other OFDM communication systems). Accordingly, the invention is not limited to application only in an LTE system, but rather is suitable for use in other communication systems as well.
The LTE uplink radio interface includes subframes, an exemplary one of which is depicted in FIG. 2a. Each subframe 200 has a time duration of 1 ms, and consists of two equally-sized slots of duration 0.5 ms. As an example, each slot can consist of seven OFDM symbols. Within one OFDM symbol, data (e.g., a number, M, of modulation symbols) are transmitted in parallel on a large number of narrowband subcarriers. As is known in the art, each OFDM symbol includes a cyclic prefix whose purpose is to make the OFDM signal insensitive to time dispersion on the radio channel.
The uplink transmission can be described as a time/frequency grid as illustrated in FIG. 2b, in which each resource element or modulation symbol corresponds to one subcarrier during one OFDM symbol interval. For an LTE system, the spacing between neighboring subcarriers is 15 kHz, and the total number of subcarriers can be as large as 1200. As also illustrated in FIG. 2b, the subcarriers are grouped into resource blocks, wherein each resource block consists of 12 subcarriers during one 0.5 ms slot. With seven OFDM symbols per slot, there is thus a total of 12×7=84 resource elements in a resource block. One such resource block is illustrated as the shaded area in FIG. 2b. 
In the LTE radio-access technology, as well as in others, the uplink radio channel of a mobile-terminal-to-network link can be estimated by means of known reference signals that are transmitted by the mobile terminal within specific DFTS-OFDM blocks. The radio channel over a bandwidth equal to the instantaneous bandwidth of the uplink data transmission can be estimated by means of the transmission of so-called “demodulation reference signals”, transmitted within the fourth OFDM symbol of each slot. Of note is the fact that each demodulation reference signal has a bandwidth equal to the bandwidth of the data transmission. The situation for two exemplary OFDM slots is illustrated in FIG. 3. These reference signals can, for example, be used for channel estimation for coherent detection of the uplink data transmission from the mobile terminal.
The demodulation reference signals can, in the frequency domain, be seen as consisting of a number of subcarriers. Generation of the demodulation reference signals typically is by means of “normal” OFDM processing (i.e., no DFT precoding is used).
Generally speaking, cellular systems suffer from co-channel interference, whereby simultaneous transmissions use the same physical resources and thus generate mutual interference. This co-channel interference reduces the signal quality (e.g., measurable as a signal to interference plus noise ratio—“SINR”) and in turn reduces the system capacity. Systems having a dense deployment of nodes are especially interference-limited, meaning that their performance is limited by co-channel interference.
A technique called “coordinated multipoint reception” (CMPRX) is being considered for use in systems such as LTE-Advanced because it is a promising technology for improving the system-level performance in the uplink direction (i.e., from a user equipment—“UE”—to a base station or eNodeB) in interference-limited scenarios. The basic idea of CMPRX is to allow a baseband receiver to use antennas situated at multiple sites to demodulate the symbols transmitted by various UEs on the uplink. One implementation of CMPRX, illustrated by the arrangement depicted in FIG. 4, comprises an eNodeB 401 that is connected (e.g., by means of fiber cable 403) to multiple antennas 405 located at different sites. This eNodeB 401 acts as a “coordination center” 401 having a geographic coverage area referred to as a Distributed Antenna System (DAS) cell 407. A number of UEs can be present and served by the coordination center 403. In the illustrated embodiment, there are three of them (UE 409-1, UE 409-1, and UE 409-3), although it will be appreciated that at any given time there could be more or fewer UEs. In this arrangement, the signals transmitted by each of the UEs 409-1, 409-2, 409-3 in the DAS cell 407 are demodulated together using all of the network antennas 405 within the coverage area of the DAS cell 407. It will be appreciated by those skilled in the art that the present invention will be equally applicable to scenarios in which all the receive antennas (i.e., antennas on the network) belonging to one DAS cell are located at one site. In such scenarios, the DAS cell becomes an ordinary cell in which multiple UEs are allowed to transmit simultaneously on the same set of subcarriers.
Two particularly attractive baseband techniques for demodulating the signals received from the UEs 409-1, 409-2, 409-3 in each DAS cell 407 are: successive interference cancellation (SIC) and interference rejection combining (IRC). Each of these baseband receiver techniques requires that the channel between each mobile and each receive antenna be estimated by the uplink receiver. It has been shown that the quality of these channel estimates greatly influences the performance of SIC as well as IRC.
As mentioned earlier with reference to FIG. 3, uplink channels are typically estimated by the uplink receiver from demodulation reference signals (RSs) that are transmitted from each UE antenna. (Modem UEs are often designed having two or more transmit antennas to improve transmitter and receiver performance.) In LTE systems, one OFDM symbol out of each 0.5 ms slot is devoted to the transmission of an RS by all UEs. Hence, when estimating any given uplink channel at the coordination center 401, the other reference signals act as interference, which degrades the accuracy of the channel estimate. As the interference among the different RSs increases the channel estimation quality decreases. To mitigate this effect, the reference signals used by all of the UEs being served by one DAS cell will ideally be orthogonal with respect to one another.
For example, consider a system in which each UE is equipped with Ntx transmit antenna ports. The term “antenna port” is used here instead of the term “transmit antenna” in recognition of the fact that several physical transmitting antennas can be configured such that they appear as one antenna from the perspective of the receiver. The term “antenna port”, then, is intended to cover all possible embodiments, including a single physical antenna as well as two or more antennas configured to act in concert so as to be equivalent to a single transmitting antenna from the point of view of a receiver. Note that in the LTE standard, every downlink transmission is always expressed as being carried out from a set of antenna ports.
Assume that a number, NUE, of UEs can be simultaneously served within one DAS cell. Thus, ideally, it would be desirable to have (Ntx*NUE) orthogonal reference signals available for use in each DAS cell.
To take a numerical example, consider a DAS cell having 7 antenna sites (which corresponds to 21 sectors). Furthermore, assume that each sector will serve a maximum of one UE. This means that the maximum number of UEs that can be simultaneously served by this DAS cell is equal to NUE=21. If each of the UEs has Ntx=2 transmit antenna ports, each DAS cell would require Ntx*NUE=2*21=42 orthogonal reference signals. However, current system designs often allocate fewer orthogonal reference signals per DAS cell. For example, the present LTE Release 8 standard supports having 8 orthogonal reference signals within each DAS cell.
Increasing the number of orthogonal reference signals inherently requires devoting more uplink resources to the transmission of uplink reference signals, and this in turn reduces the amount of uplink resources left for transmission of data. This suggests that, ideally, one would like to have as few orthogonal reference signals on the uplink as possible.
In present systems, the number of uplink orthogonal reference signals needed in a DAS cell increases by Ntx for every additional UE that is served by the DAS cell. Hence, with NUE UEs being served by a coordination center, there will be a need to set aside enough uplink resources to support Ntx*NUE orthogonal reference signals. As NUE and Ntx become large, a substantial portion of uplink resources could be taken away by the reference signals alone, making these resources unavailable for transmitting uplink data.
It is therefore desirable to provide methods and apparatuses that allow each DAS cell to operate with fewer than Ntx*NUE orthogonal reference signals while minimally degrading the performance experienced by each UE within the coordination center's coverage area.